This does work in contrast (I simply output the key {force_update_heartbeat}): https://svelte.dev/repl/d2358d0e2e9c4052afad3ec85140802b?version=3.42.4
So it seems like Svelte does not add the key to the observed properties when inside each. But if it is there anyway the update works.

Edit: even with the bug fixed your REPL doesn't look Svelte-like to me. You are basically imperatively controlling the reactivity by setting force_update_heartbeat just to call foo(). If you can show a more real world example I'm sure there is a better way. Why not update the array items directly each tick? https://svelte.dev/repl/dc3ac91d693a466693a1935a96d06c73?version=3.42.4

it doesn't matter that this is code that has some real world usage, the point is that it doesn't work as expected, and this is clearly BUG.

even with the bug fixed your REPL doesn't look Svelte-like to me.

for me it's valid Svelte as well as for Svelte's compiler itself.

I 100% agree that this is a bug and I didn't say otherwise. I'm just trying to advocate patterns that fit Svelte better than the classic approach of imperatively updating everything (jQuery, Backbone, even React to some degree).

@Prinzhorn

More real life example and very close to what I'm trying to do:

https://svelte.dev/repl/f5df58cacfba4c71a81b3072ae4ad955?version=3.42.4

It works when #key is outside of #each.

I don't want to store the evaluated data, "time passed" in the example" and just display it using the returned value of a function. And I'm using #key because that's a hacky way of re-evaluating the function when the expression passed to #key changes.

Besides this point, as said in the documentation (#key section link):

This is useful if you want an element to play its transition whenever a value changes.

So I also included the transition examples. Transition when #key is outside of #each but doesn't otherwise.

And I don't know what's going on but transition looks also buggy.

@chulman444

Here's a more Svelte-like version with a now store (ask me about stores, I love stores. Have you heard about stores?) https://svelte.dev/repl/1ce622ca561e41fe817c4b10d7351059?version=3.42.4

Edit: in a real application I'd suggest having now run just a couple of seconds ahead. Otherwise if you post an item literally now, it might show "in 1s" and not "right now" or "a moment ago" etc. because now is already outdated

Also doesn't work inside {#if} and {:else}:
https://svelte.dev/repl/f7587129060b4e7eb19d6684832c963f?version=3.43.0

Update the issue title?

Also doesn't work inside {#if} and {:else}: https://svelte.dev/repl/f7587129060b4e7eb19d6684832c963f?version=3.43.0

Update the issue title?

I spent a lot of time wondering why the {#key} block I was trying to use did not work as expected. I just now found this issue and I happened to be using it inside an if. I agree that the issue should be renamed so others can find it

A workaround I'm using is to add bind:this={foo} in the element inside the {#key} block. No idea why it works though.
My real world use case is to trigger an animation (out of several selected by #if) when a value changes.

pretty sure this was resolved in #7422

https://svelte.dev/repl/d4112f1ae05046bdb48be42b62cc4c8d?version=3.47.0

It was, thanks for noticing!